This invention relates generally to digital multimeters, and in particular to a multimeter having a filtered measurement mode.
Digital multimeters measure a wide range of electrical parameters, such as a.c. and d.c. currents and voltages, and resistance as well as physical parameters that can be converted to electrical signals, such as temperature, capacitance, and frequency of events. One commercially available family of digital multimeters favored and used by professional electricians is the Fluke 80 Series. These multimeters have additional measurement capabilities, including true RMS (root-mean-square) voltage measurement, freezing a measurement in a so-called “hold” mode, measuring the minimum and maximum (Min/Max) voltages of a varying signal, and making diode checks. Because these multimeters are capable of accurate RMS voltage measurements over a broad band of frequencies, they are a versatile tool for investigation of power quality problems, such as transients and harmonics.
However, there are a number of measurement situations confronting the technician in which it is extremely difficult to make accurate measurements. For example, modern adjustable speed motor drives, uninterruptible power sources, and switching power supplies use pulse-width modulation using high frequency, high voltage pulses to generate pulse-width modulated pulses, or lower frequency sinusoidal signals, with higher frequency components in the form of switching transients and voltage spikes superimposed on the signals. The resultant RMS voltage measurement values read erroneously higher than they should because total energy of the combined outputs is being measured.
It would be desirable to add a measurement feature in which switching transients and voltage spikes are filtered for low-frequency measurements without compromising the broadband measurement capabilities of the multimeter.